1. Field of the Invention
The present invention relates to high thermal conducting mixtures which are used to make high thermal conducting mixes which are used to make high thermal conducting formulations, one formulation of which has particular use as a heat transfer means for cooling electronic components such as VLSI chips.
2. Description of Related Art
Electronic devices and components generate heat and in many applications the heat needs to be effectively dissipated for the device to function. In one example, integrated circuit chips which are mounted in an array on substrates need to achieve greater computer speeds and the miniaturization of electronic devices has resulted in components which generate more heat and which require denser packing of the active components of the electronic device. The use of such components and denser packing is only practicable, however, when the heat generated is effectively removed from the active components and a number of techniques have been developed to cool such electronic components.
Liquid coolants are commonly employed in such high density cooling applications and various systems have been disclosed in the art for providing a separate cooling member to cool the component. Fins, channels, baffles, and other heat exchanger type devices have been developed to remove heat from the electronic component. Exemplary apparatus for cooling integrated circuit chips are shown in U.S. Pat. Nos. 5,177,667 and 5,294,830, the disclosures of both patents being herein incorporated by reference.
In most of these cooling devices there is an air space between the electronic component to be cooled and the cooling device which air space has a relatively low heat transfer rate. To overcome this problem, thermal compounds or formulations commonly referred to as thermal greases or pastes, gels, adhesives, and composites have been developed to enhance the transfer of heat from the electronic component to the cooling device by contacting and connecting both surfaces and providing a high heat transfer conducting means. The following description will be directed to thermal pastes for electronic cooling devices although the invention is applicable to make other products such as cosmetic, pharmaceutical, and automotive products.
The thermal compound must have high thermal conductivity and preferably be an electrical insulator. The compound must also have a low viscosity and be compliant so that the thermal compound can be easily applied to the surfaces of the electronic component to be cooled and can conform with the microscopically rough surface of the electronic component which are often bowed or tilted to minimize air gaps which are detrimental to the cooling process. Low viscosity is also important because the electronic components and the solid bonds that attach the components to a substrate are fragile and the force needed to apply the thermal compound must be minimal.
A number of thermal compounds have been developed to be used in electronic systems and, in general, the thermal compound comprises thermally conductive particles in a curable or non-curable liquid dielectric medium along with other components such as dispersants, antioxidants, and stabilizers.
U.S. Pat. Nos. 5,094,769 and 5,213,704 provide a phase-stable, thermally conductive paste having a relatively constant thermal conductivity and viscosity for transferring heat from electronic components. A critical component of the paste is the use of a coupling agent such as an organosilane which has a functionality which is reactive with the thermally conductive particles and a functionality which is of similar polarity to the liquid carrier. The disclosures of these patents are hereby incorporated by reference.
In U.S. Pat. No. 5,098,609 stable high solids, high thermal conductivity pastes are disclosed which contain a stabilizing dispersant to inhibit liquid-solids separation and which facilitate incorporation of more thermally conductive solids into the paste. Examples of stabilizing dispersants are monomeric acids; monomeric, oligomeric and/or polymeric fatty acid esters and salts thereof; petroleum sulfonates; and polyimides. Polyesters as disclosed in U.S. Pat. No. 3,778,287 to Stansfield and Hypermers made by ICI are disclosed in the patent as being useful as the stabilizing dispersant. The disclosures of both patents are hereby incorporated by reference.
Increasing demands of industry however, are requiring thermal compounds having improved thermal conductivity in order to be used with the new computers and other electronic components now being developed. In general, higher thermal conductive material loadings are needed for higher thermal conductivity compounds and while compounds having thermal conductivities ranging from 1-6 W/mK (watts/meter-degree K) are commercially available, the pastes tend to be too thick and stiff at the high solid loadings required for thermal conductivities greater than 3 W/mK. Hard, non-compliant and difficult to dispense pastes cannot be used for most applications.